You've got mail

by Gary Kath and Craig Bishop, Scotch Plains, NJ -- EDN, 8/19/1999

Many e-mail programs provide a "beep" or a pop-up message box signaling the user that a new e-mail message has arrived. If the you are too far from the computer to hear the audible signal or if the monitor is turned off, then you miss the new-mail audible and visual signals. The simple circuit in Figure 1 latches on an LED and an audio sounder when an appropriate new audible e-mail signal occurs. The method replaces the normal e-mail sound.wav file with a .wav file of any valid recorded dual-tone multifrequency (DTMF) sound. The circuit listens for the DTMF tone and latches on LED D₁ and the piezoelectric buzzer, P₁. R₁ biases the microphone's FET, and C₁ couples the audio to the M-8870 DTMF-receiver, IC₁ (Teltone Corp, www.teltone.com). IC₁ integrates both bandsplit-filter and decoder functions into an 18-pin DIP.

Resistors R₂ and R₃ configure the on-chip differential amplifier for a gain of 47. The 3.579545-MHz crystal, X₁, provides a precise clock generator for IC₁'s digital-counting decoding circuitry. R₄ and C₂ provide an RC guard time to place accept and reject limits on tone duration. IC₁'s STD output switches to logic high for the duration of any valid DTMF tone. The NAND gate, IC_2A, inverts this logic signal and then directs it to a latch configured with NAND gates IC_2B and IC_2C. Pushbutton switch S₁ resets the latch. IC_2D buffers and inverts the latch's output and drives a 2N2222 transistor, Q₁, thereby turning on the LED and the piezo buzzer. The circuit latches for any valid DTMF tone. You can add additional circuitry to use the M-8870's 4-bit binary outputs, Q₁ to Q₄ if you need to discriminate between DTMF tones. (DI #2399).